Size-selective encapsulation of C 60 and C 60 -derivatives within an adaptable naphthalene-based tetragonal prismatic supramolecular nanocapsule

Novel naphthalene-based 5·(BArF) 8 capsule allows for the size-selective inclusion of C 60 from fullerene mixtures. Its size selectivity towards C 60 has been rationalized by its dynamic adaptability in solution that has been investigated by Molecular Dynamics. Additionally, 5·(BArF) 8 encapsulates C 60 -derivatives such as C 60 -PCBM and N-methyl-pyrrolidine-C 60 . The latter can be separated from C 60 since 5·(BArF) 8 displays distinct affinity between them

specific fullerene cage is still a challenging task basically accomplished by means of high performance liquid chromatography (HPLC) techniques, that are generally tedious, time-and energy consuming. 8 Another challenging issue is the separation of parent fullerenes from their corresponding derivatives, which is also mainly restricted to HPLC separation. C60-derivatives such as [6,6]-phenyl C61 butyric acid methyl ester (PCBM-C60) or fulleropyrrolidines, present promising applications in material science; but they need to be available in a pure form to display full efficiency. 9,10 To date, numerous examples of 3D molecular receptors based in metal-ligand coordination bonds and capable of hosting fullerenes have been reported. 11-14 However, the separation of a single fullerene by size-confinement effects is not straightforward since the molecular receptors display high affinity for different-sized fullerenes simultaneously. 15 In most reported examples a higher affinity towards Cn (n ≥70) over C60 is observed, due to more significant solvophovic effects and the more extended π-surface of larger fullerenes that enhances capsulefullerene interaction. [16][17][18][19] The reverse trend is observed in very limited examples. 20 Our group previously reported a coordination nanocapsule (4·(BArF)8) capable of encapsulating different sized fullerenes from C60 to C84. 21 This lack of size selectivity was attributed to its pronounced breathing ability and large cavity size. In light of the importance of C60 and its derivatives, herein we report a nanocapsule (5·(BArF)8), which bears a smaller cavity, obtained by tuning the length of its spacers. The newly designed nanocapsule with naphthalene spacers is capable of exclusively encapsulating smaller C60 fullerene and its corresponding mono-adduct derivatives in a selective manner.
Crystallographic data was obtained from 5·(CF3SO3)8 crystals at the XALOC beamline of the ALBA Synchrotron ( Figure 2a and Supplementary Information for Single-Crystal X-Ray Diffraction -SCXRD details). Nanocapsule 5 8+ consists of two parallel tetracarboxylated Zn II -porphyrins linked by four macrocyclic dinuclear Pd II complexes. The four-carboxylate residues of each porphyrin are linked by means of η 1 -O monodentate coordination to one Pd II center ( Figure S38). As in 3 8+ and 4 8+ , [19,20] 5 8+ presents a tetragonal prismatic geometry bearing a D4 symmetry. The crystallographic PorphZn···PorphZn distance in 5·(CF3SO3)8 is 8.2 Å. As expected, this distance lays between the observed in capsules 3·(X')8 and 4·(X)8 (see Scheme 1 and Figure 2a). In light of the size contraction with respect to 4·(X)8, it was predicted that 5·(X)8 will offer a higher affinity to smaller fullerenes.
Once 5·(BArF)8 was fully characterized, its ability to host fullerenes was tested starting with the encapsulation of C60. Here, fast formation of 1:1 host:guest adducts was observed after mixing a 1:1 molar  Figure S40). 23 The formation of the host:guest adduct was also evidenced in the 1 H-NMR spectrum, which showed several signals shifted; specially those corresponding to the aromatic protons of the phenyl rings of the porphyrin pointing inwards the cavity ( Figure S41).
Afterwards, the encapsulation of C70 was also attempted and monitored by HRMS. Remarkably, when a solution of C70 in toluene was mixed with a solution of 5·(BArF)8 in CH3CN in a 2:1 molar ratio (298 K, 48 h), peaks corresponding to C70⸦5·(BArF)8 adduct as well as peaks belonging to remaining empty capsule were observed (3:2 ratio respectively, see Figure S42). These results indicated a notable lower affinity of the capsule towards C70 compared to C60. UV-Vis titration with C70 was in line with HRMS, since negligible changes on the Soret band were detected (Fig.S43).
The encapsulation of C60 might seem surprising based solely on the short Porph-Zn···Porph-Zn distance measured in the crystal structure. This distance, 8.2 Å, is much smaller than the van der Waals diameter of  (Figure 2b). Indeed, when comparing these Porph-Zn···Porph-Zn distances to those observed in the previously reported crystal structures of C60⸦4·(BArF)8 and C70⸦4·(BArF)8, 13.1 Å and 13.7 Å for C60 and C70 respectively, 21 the idea that C 70 fullerene is imposing a more severe distortion on the nanocapsule 5 than C 60 is reinforced. This large distortion of the nanocapsule 5 required to encapsulate C70 is in agreement with the hampered encapsulation of C70 within 5·(BArF)8 and the higher selectivity towards C60.
Encapsulation experiments with C60 were also performed by using 5·(CF3SO3)8 in the solid state.
Interestingly, C60 was trapped after adding solid capsule to a solution of the fullerene in toluene (1:8 capsule:fullerene molar ratio) ( Figure S45). Full formation of C60⸦5·(CF3SO3)8 adduct was observed after 1 h stirring. Note here that the liquid/liquid complexation occurs faster (<5 min, 1:1 capsule:fullerene ratio) than the solid/liquid one. The slower rate of the solid/liquid encapsulation is most likely due to the higher rigidity of the capsule in the solid state, which restricts its dynamic adaptability needed to accommodate C60, as revealed by MD simulations.
Taking advantage of the higher size-selectivity for C60, nanocapsule 5·(BArF)8 was used to selectively separate C60 from fullerene extract (extract composition: 70% C60, 28% C70, 2% higher fullerenes). To this end, a solution of 5·(BArF)8 in CH3CN was mixed with a solution of fullerene extract in toluene, in a 1:2 molar ratio (capsule:fullerenes). Delightfully, HRMS experiments showed that C60 was exclusively encapsulated from all the fullerenes mixture ( Figure S46). No peaks corresponding to adducts with C70 or with other higher fullerenes present in the soot (i.e. C72, C76 or C84) were observed, even using extended reaction times. The same experiment was repeated using solid 5·(CF3SO3)8, in which C60 fullerene was also solely encapsulated (capsule:fullerene-extract in 1:8 molar ratio, see Figure S47). On the contrary, when larger 4·(BArF)8 capsule is added, either in solid or liquid form, to a fullerene extract solution both C60 and C70 and other higher fullerenes are encapsulated displaying no size selectivity. 21   Finally, the ability of 5·(BArF)8 to encapsulate C60 derivatives was also explored. Host-guest studies were carried out with PCBM-C60 and N-methylpyrrolidine-C60 (the latter was prepared as previously reported). 24 When a solution of the corresponding derivative in toluene was mixed with a solution of the capsule in acetonitrile in a 1:1 molar ration, clean formation of the host-guest complexes was observed by HRMS after 5 min stirring (Figure 1c-d, Figure S48 however, encapsulation of PCBM-C60 was faster as it was evidenced by the HRMS spectrum recorded after 30 min reaction time compared to the one obtained for N-methylpyrrolidine-C60 (Figures S52 and S53).
The latter observation is in agreement with the higher association constant obtained for PCBM-C60.
A competition experiment between C60 fullerene and PCBM-C60 derivative, further supports the similar values obtained for their association constants. When 1 equivalent of 5·(BArF)8 capsule was mixed with 2.5 eq of C60 and 2.5 eq of PCBM-C60, in toluene/acetonitrile (4/1) during 2.5h, very similar peaks on the HRMS were observed in line with the similar Ka (see Figure 3a and Figure S54). Interestingly, when a solution of 1 equivalent of 5·(BArF)8 capsule was mixed with 2.5 eq of C60 and 2.5 eq of Nmethylpyrrolidine-C60, in toluene/acetonitrile (4/1) during 2.5h, selective encapsulation of C60 occurred.